1. Field of the Invention
The present invention relates to a method for manufacturing an electroluminescence light emitting device used for a planar light source or a display element (hereinafter, also referred to as a “light emitting device”).
2. Description of the Related Art
An electroluminescence light emitting device has a light emitting layer formed using an organic compound and the like. Such an electroluminescence light emitting device is attracting attention for realizing a large-area display element at low driving voltage.
To improve efficiency of an element, Tang et al. proposed a structure in which organic compounds having different carrier transporting properties are stacked to inject holes and electrons from an anode electrode layer and a cathode electrode layer with good balance. Further, a thickness of an organic layer is set to be 200 nm or less to realize light emitting luminance of 1,000 cd/m2 and external quantum efficiency of 1% at applied voltage of 10 V or less (for example, non-patent document 1).
In developing such a high-efficiency element, it has been recognized that a technique for injecting electrons from a cathode electrode layer or holes from an anode electrode layer to an organic layer without generating an energy barrier, is an essential element.
Kido et al. proposed that a hole injecting layer is formed by a mixed layer of metal oxide and an organic compound. According to Kido et al., this can reduce driving voltage of an element and drastically reduce risk of electrical short between a cathode electrode layer and an anode electrode layer by adjusting a thickness of the hole transporting layer without increasing driving voltage (patent document 1).
However, there has been a problem of shortening luminance half life in the above described structures (patent document 2).    [Non patent document 1]: Applied Physics Letter., 51, 913 (1987)    [Patent document 1]: Japanese Patent Application Laid-Open No. 2005-123095    [Patent document 2]: Japanese Patent Application Laid-Open No. 2005-166641    [Patent document 3]: Japanese Patent Application Laid-Open No. 2000-68068    [Patent document 4]: Japanese Patent Application Laid-Open No. Hei 11-8065